Mobile device manufacturers still seem to think that adding a default signature, letting the world know what device you sent an e-mail from, is a good idea.
Step 1: From the inbox of your Kindle Fire, go to Settings.
Step 2: Under the "Sending mail" section, select Composition defaults.
Step 3: Remove or edit the signature from the text box. If you prefer to turn off signatures, uncheck the Use Signature box.
That's it. If you're considering keeping the signature, read Al Sacco's post at CIO.com on why you shouldn't.